1. Field of the Invention
The present invention relates to a constant voltage circuit used in a bias circuit such as an ECL (emitter-coupled logic) gate array and, more particularly, to a bandgap type constant voltage circuit.
2. Description of the Related Art
FIG. 1 shows a conventional Widlar type bandgap circuit 30 used in a constant voltage circuit. In the bandgap circuit 30, a collector-emitter path of a first npn transistor Q1, a first resistor R1, a collector-emitter path of a second npn transistor Q2, and a second resistor R2 are connected in series with each other between a ground voltage GND and a negative source voltage V.sub.EE in the order named.
A third resistor R3 and a collector-emitter path of a third npn transistor Q3, the collector and base of which are connected to each other, are connected in series with each other between the emitter (output terminal) of the first transistor Q1 and the negative source voltage V.sub.EE in the order named.
In addition, a fourth resistor R4 and a collector-emitter path of a fourth npn transistor Q4 are connected in series with each other between the ground voltage GND and the negative source voltage V.sub.EE in the order named. The collector of the fourth npn transistor Q4 is connected to the base of the first npn transistor Q1, and the base of the fourth npn transistor Q4 is connected to the collector of the second npn transistor Q2. The collector and base of the third npn transistor Q3 are connected to the base of the second transistor Q2.
In the above-described bandgap circuit 30, a difference .DELTA.V.sub.BE between a voltage of a base-emitter path of the transistor Q3 and a voltage of a base-emitter path of the transistor Q2 appear across the resistor R2. A voltage difference .DELTA.V.sub.BE is multiplied with R1/R2, and the product appears across the resistor R1. The sum of the voltage .DELTA.V.sub.BE .multidot.R1/R2 across the resistor R1 and a voltage of a base-emitter path of the transistor Q4 .DELTA.V.sub.BE4, that is, EQU (.DELTA.V.sub.BE .multidot.R1/R2)+V.sub.BE4 (1)
is an output voltage V.sub.ref. Since the first term of equation (1) has a positive temperature coefficient, and the second term has a negative temperature coefficient, by adjusting the value of the resistor R1, a constant voltage output having a temperature coefficient of zero can be obtained. The value of the output voltage V.sub.ref with respect to the negative source voltage V.sub.EE is stabilized. When the output current increases, a current flowing in the resistor R1 decreases. The base current of the transistor Q4 is decreased by the decrease in current flowing in the resistor R1. Then, the collector current of the transistor Q4 decreases. When the collector current of the transistor Q4 decreases, the base current of the transistor Q1 increases.
In the bandgap circuit 30 described above, however, the negative feedback function by the transistor Q4 against variations in negative source voltage V.sub.EE is not always sufficient, so that variations in output voltage V.sub.ref do not sufficiently follow those in negative source voltage V.sub.EE. Therefore, the difference between the output voltage V.sub.ref and the negative source voltage V.sub.EE is not kept constant, so that the output current flowing into the load side unexpectedly varies.
As described above, in the conventional bandgap circuit, a constant voltage output can be obtained against variations in temperature. The negative feedback function by the transistor Q4 is, however, not always sufficient against the variations in negative source voltage V.sub.EE. Therefore, the variations in output voltage V.sub.ref do not sufficiently follow those in negative source voltage V.sub.EE, the difference between the output voltage V.sub.ref and the negative source voltage V.sub.EE is not kept constant, and the output current flowing into the load side unexpectedly varies, resulting in inconvenience.